Maskless optical interferometric lithography

ABSTRACT

An optical device generated by interferometric lithography, in the form of a surface relief pattern that diffracts light, to be used mainly in optical security, produced by optical systems compliant with the Schimpflug and hinge Rules containing: two identical lenses L 1  and L 2 , two physical object location B 1  e B 2  and one image plane A 1 ≡A 2 . No physical mask is needed throughout the origination process, thus eliminating any kind of border effects. The device actually implements an in-plane focused hologram, with a complex surface pattern of valleys and ridges with a non-trivial generating function, which is, by itself, both an additional security feature of the device and its fingerprint. The time to generate the optical device is proportional to the number of colours specified for the reference geometry and not depend on the overall area of the optical device.

The present application is a National Phase Application of PCTInternational Application PCT/PT2003/000004, entitled “Maskless OpticalInterferometric Lithography” filed on Mar. 25, 2003, which isincorporated herein by reference in its entirety.

TECHNICAL DOMAIN OF THE INVENTION

This invention addresses the optical origination, throughinterferometric lithography, of optical elements endowed with optimumcharacteristics to securely protect products and documents againstforgery and counterfeiting.

BACKGROUND OF THE INVENTION

Interferometric lithography is one of the most flexible tools togenerate microstructures that, by diffraction, produce polychromaticbeams with spectral and angular properties of importance for opticalsecurity devices [11].

The way such microstructure diffracts light depends, on the one hand, onthe shape, dimension and position of the light sources and, on the otherhand, on the shape and dimensions of the spatial domain whereinterference takes place [12 and 13].

Many implementations of interferometric lithography have been described.The most important are:

-   point light sources producing spherical divergent waves, or plane    waves obtained after collimation, the position and orientation of    which can be controlled, the interference pattern being confined to    the interior of a mask placed as close as possible to the plane of    the photosensitive emulsion, typically of photographic nature;-   two spherical waves interfering on their common focal volumes, said    waves being formatted and shaped by two or more lenses properly    located and adequately phased.

In the first case, a non-pixelated pattern is created. This type ofimplementation can be found in patents [3 to 6]. In the second case, theinterference is restricted to dots of controlled shape, the pixels,usually arranged in random, polar or rectangular format. This type ofimplementation can be found in patents [7 to 10]. In the pixelated case,there is an inefficient utilisation of the photosensitive area, as thequality of the interference pattern—consequently the micro-relief thatis generated after chemical or thermal processing—strongly depends onthe Gaussian character of the laser beam. It is possible to overcomethis problem by masking the Gaussian beam, using only the central partof it, but, in this case, the amount of available energy to expose theemulsion is reduced, leading to longer exposures. Visually, unless thedensity of dots is very high, the pixelated character is always visible,even if it does not jeopardise the quality and the security of thedevice. The larger the density of pixels, the longer will be the timeneeded to complete the exposure of all pixels that build up the desiredshape. Manufacturing time is thus proportional to the total number ofpixels.

In both cases, the interference pattern is very simple, basicallyconsisting of quadratic fringes that can be approximated in most of thecases by linear and parallel fringes. Diffraction by linear phasegratings is well known, and the optical effects that can be obtained arelimited [12].

In-plane holography (or focused holography) is a technique to encodeimages holographically, by creating an in-focus real image (object beam)on the photosensitive plane and introducing a reference beam—typicallyplane or spherical—thus generating an interference pattern within thearea covered by the object beam [14]. On reconstruction, the spectraland angular properties of the diffractive beam are of interest. Theproblem with focused holography is that if the area of interest is to becovered by a several patterns, say N, each point receives energy fromthe reference beam N times, which severely degrades the modulation ofthe microstructure and reduces the diffraction efficiency, thus actuallydestroying it as a security device. If, somehow, the reference beamcould be restricted to the exact area of the object beam, this problemwould be overcome. One way to solve the problem is to place a mask withthe desired shape in the image plane, as close as possible to theemulsion to avoid diffraction problems. The problem with this techniqueis that all the masks for the different patterns must be preciselyaligned and separated from the emulsion with a thin layer of an opticalliquid (to match the refractive index), making it very difficult (oreven impossible) to use when a large number of masks is required. Thetotal amount of time needed to complete the creation of the opticalelement is, in this case, proportional to the number of patterns (i.e.masks) needed to define the element image

INVENTION EXPLANATION

This invention shows how implement optical systems that ensure focusedholography for an arbitrary number of polygons that fully cover thefinal desired image (composed of several patterns), using interferencebeams making an adequate angle, without the need of physical maskslocated near the image plane, while ensuring a processing timeproportional to the number of colours specified for the referencegeometry.

One embodiment includes a maskless optical setup to generateinterference patterns within selected areas of a photosensitivematerial, without the need of any physical mask to delimit or delimitatethe spatial extent of the register of the interference pattern, ensuringthat the surrounding area is not affected by light. The device mayensure that the time needed to register all the design polygonalpatterns (that build up the complete optical device) is linearlyproportional to the number of colors specified for the referencegeometry and not to the area or to the number of pixels within theoverall area of the optical device. The device may be based on theScheimpflug and Hinge conditions, ensuring adequate superpositionbetween different optical beams in an imaging configuration. The deviceoptical configuration may be based on two object physical locations andtwo optical channels. The device optical configuration may be based onone object physical location and two optical channels. The deviceoptical configuration may be based on one object physical location andone optical channel.

The invention is based on a well known principle of photography todetermine how a viewing camera focuses when both the object and theimage plane are tilted with respect to the optical axis of the lens: theSchempflug Rule [1, 2 and 15]. This rule, which is a necessary conditionfor focusing—meaning that there are many ways to adjust the camerasatisfying the rule but not ensuring focusing—needs to be complementedby a second necessary condition, the Hinge Rule: when both rules aresatisfied, the camera will be in focus [2]. As it is well known,diffraction effects are minimised when the image is precisely focused.

The Schimpflug Rule is based on Desargues theorem [16] and states thatthe object plane, the image plane and the “lens plane” should intersecton a common line. The Hinge Rule is similar, stating that the objectplane, the plane through the “centre” of the lens and parallel to theimage plane and the object focal plane should intersect along a commonline.

Mathematically, it is possible to obtain from FIG. 1 the followingequations, the first representing the Schimpflug Rule and the second theHinge Rule:

$\frac{\tan(\alpha)}{\tan(\beta)} = {{\frac{s_{0}^{\prime}}{s_{0}}\mspace{121mu}\frac{\tan(\alpha)}{\tan(\beta)}} = \frac{f}{s_{0} - f}}$β and α are the tilting angles of the object and image planes withrespect to the normal to the optical axis of the corresponding lenssubsystem, s₀ and s′₀ are the object and image distances, respectively,and the focal length of the lens.

If the two equations are combined, the lens equation [17] is obtained,showing that when both rules are satisfied, the camera is focused.

These simple rules must be slightly modified for thick lenses withnon-superposed object and image principal planes. It can be easilydemonstrated, using the lens equation, that:

-   tilted object and image conjugated planes intersect the object and    image principal planes at the same distance from the optical axis,    thus effectively splitting the Scheimpflug line into two parallel    lines, one in each principal plane (FIG. 2).-   the tilted object plane, the object focal plane and the plane    parallel to the image plane through the object principal point all    intersect on the object Hinge line; in addition, the tilted image    plane, the image focal plane and the plane parallel to the object    plane through the image principal point intersect on the image Hinge    line; both Hinge lines are parallel; they will be at the same    distance from the optical axis when the axial magnification is    m₀=−s′₀/s₀=−1 (FIG. 3).

In a tilted image plane configuration, the magnification is not uniformacross the field. For example, in order to get similar image and objectdimensions, the axial magnification should be set to m=−1 by ensuringthat both axial object and image distances are set to 2f, f being thefocal length. When the image and object planes are tilted, as we moveaway from the optical axis, the magnification changes because the objectpoint and image point distances vary.

The known non-uniform magnification can be compensated by apre-distortion of the object input, at the object plane. Thiscompensation can be made digitally. It can be shown that themagnification between tilted conjugated planes is described by (FIG. 4):

${m\left( {x^{\prime},\alpha} \right)} = {m_{0} + \frac{x^{\prime} \cdot {\sin(\alpha)}}{f}}$m₀ is the axial magnification, x′ is the in-plane distance from theoptical axis, f the focal length and the tilting angle of the imageplane with respect to the normal to the optical axis of thecorresponding lens subsystem. For every position in the horizontal axisof the image plane, the magnification will be the same in the verticalline that crosses it.

It should be noted that centrally symmetric conjugates are notnecessary, although such a configuration minimises distortion. It may beinteresting or necessary to change the dimension of the image withrespect to the object, and distances should be established according tothe paraxial equations of geometric optics.

FIG. 5 illustrates for thin lenses one possibility to actually implementfocused holography with equal format object and reference beams; changesshould be obvious for thick lenses. Two optical channels satisfying theScheimpflug and the Hinge Rules are composed of two tilted objectphysical locations B1 and B2 and two identical lenses L1 and L2 thatcreate real superimposed images on the final common tilted image plane(A1 and A2). The two objects must have the correct orientation andpre-distortion to correct the non-uniform magnification in order tocorrectly superimpose the two images. The points S1 and S2 represent theintersection points complying with the Scheimpflug Rule.

The optical beams propagating along the two optical channels interfereand the interference pattern is recorded in a photosensitive materialplaced in the image plane (A1≡A2). From an optical point of view, itshould be stressed that the local interference angle changes from pointto point, that complex image waves (not just simple spherical waves) areinterfering and that the interference pattern is much more complex thana simple linear pattern.

The mean grating period of the interference pattern depends on the meanangle between the two optical channels. By controlling such mean angle,it is possible to control the geometry of observation and the way thedifferent wavelengths (colours) change when that geometry changes. It isalso possible to define a reference geometry (with a particularilluminant, an angle of illumination and an angle of observation) and,for that particular geometry, colours corresponding to grating periodsand orientations. The object is composed of an envelope of 2D polygons,each one with its own colour for the reference geometry. Nevertheless,the overall number of colours is always much smaller than the number ofpolygons. The different colours of all the 2D polygons result fromdifferent grating orientations. To obtain different gratingorientations, the objects and the photosensitive material in the imageplane should be rotated around the normal to the image plane.

The object can be on display on any kind of device that encodes thedesired object distribution. As stated before, with photographic masksthere is the problem of aligning every new mask. This problem isovercome if the object is displayed on a Spatial Light Modulator (SLM)that needs to be aligned only once, allowing all the exposures to takeplace in fast sequence. The system must use laser light and the way thebeam is introduced into the system depends on the nature of the display(reflective or transmissive).

In the image focal plane of each optical channel (F1 and F2),perpendicular to the optical axis of the corresponding lens, a Fourierplane is available for optical filtering, thus allowing the control ofthe object spatial frequencies by phase or amplitude filtering.

The finest detail of the final image depends only on the object finestdetail and the optical setup magnification.

In a second configuration, only one physical object location has to beselected and two images of it are created. The system is more compact,easy to align and easy to control. FIG. 6 describes the changes, using areflective SLM to display the object, and the corresponding setup.

A filtered collimated laser beam (I) enters the system. After reflectionon the plane mirror (M), the beam is redirected to the reflective SLM(B), where the object distribution is encoded, thus modulating theamplitude of the beam (I). The modulated beam is then split by beamsplitter (BS) into two perpendicular beams. A critical aligmnent isrequired to ensure that the diagonal of the BS is within the symmetryplane of the overall optical setup. Mirrors M1 and M2 redirect light tothe lens, creating two different virtual images of the object, in thesame object positions as described in FIG. 5.

As it is not possible to the change spatial orientation of one opticalchannel without changing the other for the same tilt angle, the imagegiven by one channel will be symmetrical with respect to the other. Ifthe object has horizontal symmetry the overlapping is perfect (FIG. 7a). For non-symmetric objects, it is necessary to compensate the lack ofsymmetry (FIG. 7 b), for example, by creating a new symmetric objectthat results from joining the desired object with is symmetric image(FIG. 7 c). We thus overcome the problem with the cost of reducing thesize of the original object by 50% (region of interest in FIG. 7 c).

The optical beams propagating along the two optical channels interfereand the interference pattern is recorded in a photosensitive materialplaced in a controlled rotational stage in the image plane (A). Thecentre of rotation should be the centre of the image for symmetricobjects or the centre of one of the two images for non-symmetric ones.The convenient spatial frequency filtering masks are located in theFourier planes (F1 and F2).

To control the relative intensity between the final interfering beams,it is possible to add neutral density filters. A most adequate way is touse polarised laser light and split the beam according to thepolarisation, thus using all the available energy. In FIG. 5 a filtered,polarised and collimated laser beam (I) enters the system, the halfwaveplate P1 adjusts the plane of polarisation, and the polarising beamsplitter (BS) splits the modulated beam into two perpendicular beams,with relative intensities determined by P1. In one of the opticalchannels (left channel in FIG. 6) a 45° oriented half-wave plate (P2)restores the common polarisation plane for both beams.

It is also possible to implement this invention using only one physicalchannel. FIG. 8 describes the configuration. The second optical channelis the virtual image of the real one given by the mirror M. There is nocontrol of the relative intensities and the mirror M must be very closeto the rotational stage that contains the photosensitive material placedin the image plane (A).

DRAWING DESCRIPTION

FIG. 1—Positive thin lens with focal lengths f and f′ (object andimage), an object plane A and image plane A′ making, respectively, β andα degrees with the perpendicular to the optical axis. The distances s₀and s₀′ are the axial object and image distances. The points S and Hare, respectively, the points where the Scheimpflug line and the Hingeline intercept the plane of the drawing.

FIG. 2—Same as FIG. 1 with the thin lens replaced by a thick lens withits principal planes (dashed lines) and principal points P and P′. Thepoints S and S′ are the two points where the Scheimpflug lines (imageand object) intercept the plane of the drawing.

FIG. 3—Same as FIG. 2. The points H and H′ are the two points where theHinge lines (image and object) intercepts the plane of the drawing.

FIG. 4—Variation of the magnification in the image plane as a functionof the distance to the optical axis and the tilting angle. x′ is thedistance to the optical axis in the image plane, and is the image planeinclination angle with respect to the optical axis normal.

FIG. 5—Implementation based on two optical channels satisfying theScheimpflug and the Hinge Rules with object physical locations B1 andB2, two identical lenses L1 and L2, an image plane A1≡A2, two Fourierplanes for spatial filtering F1 and F2 and the two Scheimpflug Ruleintersection points S1 and S2.

FIG. 6—Implementation based on two optical channels satisfying theScheimpflug and the Hinge Rules, with one object physical location B, afiltered and collimated polarised laser beam I, a polarising beamsplitter BS, two half-wave plates P1 and P2, three mirrors M, M1 and M2,two identical lenses L1 and L2, an image plane A, two Fourier planes forspatial filtering F1 and F2, and the two Scheimpflug Rule intersectionpoints S1 and S2.

FIG. 7—a) Overlapping of the images given by the two optical channelsfor a symmetric object; b) Overlapping of the images given by the twooptical channels for a non-symmetric object; c) Compensation of thenon-symmetry with the creation of a new symmetric object.

FIG. 8—implementation based on one optical channel satisfying theScheimpflug and the Hinge Rules with one object physical location B, alens L, an image plane A, a Fourier planes for spatial filtering F, amirror M and the Scheimpflug Rule intersection point S.

REFERENCES TO PATENTS

-   [1]—J. Carpentier, “Improvements in Enlarging or like Cameras”,    British Patent no. 1139(1901)-   [2]—T. Scheimpflug, “Improved Method and Apparatus for Systematic    Alteration or Distortion of Plane Pictures and Images by Means of    Lenses and Mirrors for Photography and for other purposes”, British    Patent no. 1196 (1904)-   [3]—H. Souparis, “Method for the production of an optically variable    image”, World Patent (WO) no. 2986 (1995)-   [4]—Suga et al, “Hologram and method of and apparatus for producing    the same”, U.S. Pat. No. 5,660,954 (1997)-   [5]—Hasegawa et al, “Holographic recording appts. e.g. for laser    beam printers”, European Patent (EP) no. 534616 (1998)-   [6]—S. McGrew, “Diffractive color and texture effects for the    graphic arts”, U.S. Pat. No. 4,918,469 (2000).-   [7]—G. Antes, “Optically variable surface pattern”, U.S. Pat. No.    5,032,003 (1991)-   [8]—S. McGrew, “Holocomposer”, U.S. Pat. No. 5,138,471 (1992)-   [9]—C. Newswanger, “Holographic diffraction grating patterns and    methods for creating the same”, U.S. Pat. No. 5,291,317 (1994).-   [10]—Lu et al, “Apparatus for Producing Dot Matrix Hologram”, U.S.    Pat. No. 6,043,913 (2000)

REFERENCES TO OTHER DOCUMENTS

-   [11]—R. L. van Renesse, “Optical Document Security”, 2nd edition,    Artech House (1998)-   [12]—R. Petit, “Electromagnetic Theory of Gratings”, Springer-Verlag    (1980)-   [13]—M. C. Hutley, “Diffraction Gratings”, Academic Press (1982)-   [14]—R. J. Collier, “Optical Holography”, Academic Press (1971)-   [15]—M. Bass, “Handbook of Optics”, 2nd edition, McGraw-Hill (1995)-   [16]—E. W. Weisstein, “The CRC Concise Encyclopedia of Mathematics”,    CRC Press (1998)-   [17]—E. Hecht, “Optics”, 4th edition, Addison-Wesley (2002)

1. A maskless optical device based on interferometric lithography togenerate an interference pattern within selected areas of aphotosensitive material ensuring that the surrounding area is notaffected by light, producing diffractive optical variable image devicesfor security, comprising: a first optical channel and a second opticalchannel at an angle for interfering with the photosensitive material;the first optical channel comprising a first imaging lens for producingan image from a first object at an image plane; the second opticalchannel comprising a second imaging lens for producing an image from asecond object at the image plane, and wherein the device complies withthe Scheimpflug rule and the Hinge rule; wherein the interferencepattern comprises the images of the first and second objects; whereinthe time needed to register a complete polygonal pattern is linearlyproportional to the number of interference patterns specified for thereference geometry and not to the overall area of the optical device. 2.The maskless optical device of claim 1 comprising an opticalconfiguration selected from the group consisting of: two physicalobjects and two physical optical channels; one physical and one virtualobject and two physical optical channels; and one physical and onevirtual object and one physical and one virtual optical channel.
 3. Themaskless optical device of claim 1 wherein the photosensitive materialis rotated to materialize diffractive patterns with different gratingorientation.
 4. The maskless optical device of claim 1 wherein the anglebetween the optical channels is controllable to materialize diffractivepatterns with different grating orientation.
 5. The maskless opticaldevice of claim 1 in which the object is displayed by an amplitudespatial light modulator.